Known subscriber line interface circuits (SLIC) are found in the central office of a telephone or telecommunication network wherein a function of the SLIC is to forward signals to and from the telephone switching network of the central office exchange from and to a plurality of an analog telephone line subscribers. Analog signals are returned by the plurality of subscribers via subscriber stations or telephone instruments at remote locations from the telephone station. The SLIC is connected to the telephone instrument through tip and ring leads which form a communication loop.
Another function of the SLIC is to supply power to a subscriber station and to transmit and receive voice signals between the digital switching network and the subscriber station. These circuits, which serve to connect the subscriber to a telephone exchange, typically include a current detector which performs several functions including detection of the subscriber going off-hook during ringing. In particular, when the subscriber is being called, alerting ringing signals (typically at 50 Hz in the U. S.) from the central office exchange are applied to the subscriber loop to operate the telephone ringer. When the subscriber goes off-hook, a DC loop current changes and the resulting loop current change is detected by the central office exchange to interrupt the ringing signals in the loop. Other features of the SLIC are battery feed, over voltage protection, ringing, signaling, hybrid, and timing.
Presently known techniques for a ringing system include using external ringing generators for balanced and unbalanced ringing or employing integrated ringing generators. However, integrated ringing generators are either integrated in the SLIC circuitry and dedicated to ringing or coupled to a SLIC having only the ability to provide balanced ringing.
When a telephone current loop carries a voice signal, the signal is generally on the order of three volts in amplitude, requiring current on the order of four milliamps in amplitude, having a bandwidth of up to 16 kHz. However, a typical ringer signal may have voltage amplitudes which may exceed 150 volts, current amplitudes up to 200 ma, and a bandwidth of 75 Hz. Combining the requirements of the two types of signals, voice signals and ringer signals, yield a requirement that the circuit between the SLIC and the telephone must function as a two-channel multiplexer switch. Known techniques such as the one described in U.S. Pat. No. 5,517,565 entitled "Integrated Ringer Relay Circuit and Method,"incorporated herein by reference in its entirety, couple a SLIC and a ringer signal to a telephone system while maintaining the isolation of the two components. This arrangement enhances the quality of the voice signal without the use of relays.
Today, the telephony ringing signal requirements are determined by two factors, the market in which the telephone ringing system is to be used and the application of the ringing system. The European market uses what is known as balanced ringing, while the U.S. market uses unbalanced ringing. Balanced ringing typically requires sine wave signals applied to the tip lead and the ring leads 180 degrees out of phase. The ring lead typically has a negative offset and the tip lead has a positive offset with the net DC value of the complementary signal being zero while the AC component may be as high as 85 Vrms. In the U.S. market, the average voltage carried on the tip lead is substantially ground while the ring lead carries AC and DC signals to affect an unbalanced telephone loop circuit.